I. Field of the Invention
The present invention relates to RF signal receivers, and more particularly, to mixer optimization for RF signal receivers with swept local oscillators such as radar warning receivers.
II. Description of Prior Art
RF receivers typically include an antenna through which RF signals are received and coupled to a mixer. The mixer also receives a local oscillator signal and mixes the received RF signals with the fundamental frequency of the local oscillator signal and/or other harmonics thereof to produce an IF signal (heterodyne) which can then be processed directly. The IF signal may first be further down-converted with a second mixer and oscillator to a second IF signal (superheterodyne) before processing. In the area of radar warning receivers, the fundamental frequency of the local oscillator signal is selected so that the harmonics correlate to the police radar bands of X, K and/or Ka such that further processing will allow for determination of whether the RF signals received are police radar signals and to alert a driver thereto.
It is useful to receive RF signals from as far away from the transmitter as possible. This can be important particularly for a radar warning receiver which has to detect the presence of police radar signals from as far a distance from the radar transmitter as possible in order to alert the user to the presence of such radar at the earliest possible opportunity. As that distance from the transmitter increases, the ability to detect and utilize the RF signals becomes dependent on many factors, including the sensitivity of the RF receiver. One factor affecting such sensitivity is conversion loss in the mixer, it being advantageous to minimize such conversion loss. During manufacture of the receiver, the power of the local oscillator providing the local oscillator signal as well as the DC bias on the mixer can be iteratively adjusted until optimized mixing occurs. Those settings are then stored in a non-volatile memory and accessed by a microcontroller or other microprocessor-based device which may be used to set the power driving the local oscillator and/or the DC bias on the mixer through digital-to-analog converters (DAC) during subsequent use of the receiver. In this context, when a radar warning receiver is manufactured, a test signal is transmitted to the receiver and the LO power and DC bias are iteratively adjusted until the maximum IF signal is generated. This is believed to correspond to minimizing the conversion loss, i.e., optimized mixing, for the particular mixer product used. Those settings are stored in a non-volatile memory accessible by the microprocessor and used to set the LO power and mixer bias when the unit is active, such as when it is mounted in the user""s vehicle and turned on to detect police radar signals.
Additionally, radar warning receivers commonly look for signals in several police radar band signals as mentioned above such as by utilizing the fundamental (i.e., the first harmonic) of the local oscillator for X band radar signals, the second harmonic of the local oscillator frequency for K band signals, and the third harmonic of the local oscillator frequency for Ka band signals. Those non-fundamental harmonics may come from the local oscillator, but are more typically induced in a non-linear mixer from the fundamental frequency oscillator signal. Both are contemplated herein with reference to non-fundamental harmonics of the local oscillator signal. It is typically required that the DC bias and LO power be iteratively adjusted for each of those bands, and that the required DAC settings for the LO power and DC bias for each band be stored in non-volatile memory at the completion of the manufacturing test procedure, for use later during normal RF signal receiving operations of the receiver.
Setting up mixer optimization by iteratively adjusting the LO power and DC bias in the manufacturing process has several drawbacks. By way of example, the equipment used to test the equipment for setting the LO power and DC bias is expensive. Moreover, the time required to iteratively adjust those values is quite time consuming, especially if it must be repeated for each band of interest. Moreover, it is not uncommon for the components in the radar warning receiver to drift over time or temperature, which means that the LO power or DC bias may drift with time or temperature during use of the unit thereby degrading sensitivity. For radar warning receivers used by consumers, it is simply not practical to periodically pull the unit from the field for retesting and recalibration to maintain optimization. Moreover, such drift may occur rapidly and in a variety of contexts, such that factory recalibration is not particularly helpful.
The present invention provides an improved mixer optimization method which can be applied not only in the manufacturing process but is self-calibrating such that it can be employed in RF receivers, and especially radar warning receivers, which are in active use in the field. To this end, and in accordance with the principles of the present invention, for a given frequency or frequency band, a predetermined current correlated thereto is used to bias the mixer, and a DC voltage of the mixer is monitored while the power of the local oscillator is adjusted until the DC voltage of the mixer is at a predetermined level correlated to that frequency or band. That LO power setting is then stored in the memory of the system to drive the local oscillator for that frequency or frequency band during normal use. Periodically, the unit may again test for mixer optimization such that the LO power may be adjusted to take into account effects due to time and temperature. Typically, the unit will operate to optimize the mixer, and will then revert to normal operation, i.e., receiving and processing radar RF signals. In some cases, the DC current biasing the mixer may be retained at the predetermined level used for optimization for such xe2x80x9cnormalxe2x80x9d operation, or the bias may be adjusted therefrom to a different level for xe2x80x9cnormalxe2x80x9d operation.
In a swept oscillator system such as is used for a radar warning receiver, the optimization method may be employed between a predetermined number of sweeps or between every sweep depending upon the timing of the system, and the needs of the designers. Also, the process may be repeated for each band by providing first, second and third respective predetermined currents for biasing the mixer with respect to the X, K, and Ka bands, for example, and the power of the local oscillator adjusted with respect to each predetermined current one by one so as to determine the three settings for the local oscillator depending upon whether X, K or Ka bands are to be swept. In this regard, a typical radar warning receiver will not necessarily sweep all three bands at once, but will instead sweep band by band. Thus, the first power setting may be used for sweeping the X band, the second when sweeping the K band, and the third when sweeping the Ka band. In some cases, the power setting of the local oscillator can not be maintained level over the wide Ka band sweep. In that situation, the Ka band is broken into two narrower segments, with each segment having its own LO power setting for mixer optimization.
The local oscillator power setting for each band may thus be easily adjusted during the manufacturing process and/or while the unit is in use to maintain maximum mixer optimization. Advantageously, the optimization method is automatically undertaken during power-up of the receiver. In that case, the settings are determined anew every time the unit is powered up so that the settings need not be retained after the unit is turned off. Hence, the LO power settings may be stored in lower cost volatile memory. Also, there is no need for the expensive and time consuming test set ups otherwise used for optimizing the mixer. Thus, costs are reduced in the manufacturing process and operation of the receiver is maintained in an optimal state while in the field.